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IQ Math question in F2812 DSP

Yue Li2
Intellectual 640 points

Hello,

I am struggling with a IQ math calculation.

I got an ADC result ranging from 0000h - 7FF8h, which is a int16 (Q16)number.

Then, my calculation is  as follows:

result = ADC * 3/ 0x7FF8.
m = 2.03/result;

For instance, if my ADC result is 0x39D7, I expect to have m = 1.50.

So I wrote the code as following
//=================================================


int16 ADC;                                //Q15
_iq m;                                     //Global_Q is 24, so _iq is _IQ24


ADC = _IQ15div(ADC,32760);              //line1
ADC = _IQ15mpy(ADC, IQ15(3));            //line2
m = _IQdiv(_IQ(2.03), _IQ15toIQ(ADC));    //line3


//=================================================

Fromt the CCS watch window, only line 1 can get the result that I expect.
From line2 till line 3, the results are weird.

Can you please point out what is wrong in my code when using the IQ math.

Thanks a lot!

  • 0
    TI__Mastermind 20850 points

    Hi,

    IQ15 is a 32 bit integer, 17 bit integer part, 15 bit fraction so IQ15(3) is actually0b11 << 15 or 17 significant bits. You doa  IQ15 multiply which again gives you a 32-bit output but then assign to ADC, a 16-bit variable so im thinking either the result is getting right shifted and truncated oryou just see the least significant word - either case its thr wrong answer.

    line 1 works because the answer is <1 so truncation still preserves the fraction part.

    You can make ADC 32-bits or  you could calculate like this

    m = 2.03/3 *0x7FF8 /ADC;

    m = _IQmpy(_IQ(0.67), _IQdiv(0x7FF8 ,_IQ15toIQ(ADC)));

    I think that should work

  • 0 in reply to Vishal_Coelho
    Intellectual 640 points
    Thank you for your reply.

    However, it still didn't work.

    I used your expression, m = _IQmpy(_IQ(0.67), _IQdiv(0x7FF8 ,_IQ15toIQ(ADC)));

    From the watch window, I can see that the problem comes from the part of _IQdiv(0x7FF8 ,_IQ15toIQ(ADC)).

    For instance, if my ADC is 0x39D7, the IQ15toIQ(ADC) yields the result of 0.45. However, after the division of _IQdiv(0x7FF8 ,_IQ15toIQ(ADC)), I have the result of 0.0043.

    Why??
  • 0 in reply to Yue Li2
    Intellectual 640 points

    Vishal,

    I think I know why.

    I changed the expression to m = _IQmpy(_IQ(0.67), _IQdiv(0x7FF8 ,ADC)), instead of , _IQdiv(0x7FF8 ,IQ15toIQ(ADC)).

     

    It works now.

    Thanks!